Radiosonde system, radiosonde system receiver and signal processing method in a radiosonde receiver

ABSTRACT

This publication discloses a radiosonde system, a radiosonde system receiver and a signal processing method for use in a radiosonde system. The radiosonde system comprises at least one radiosonde ( 1 ) with means for acquisition of position and/or weather data and means for downstream transmission of the data over a radio-frequency path, signal capture means ( 2, 3 ) for reception of the weather and/or position data, and receiver means ( 4 ) for processing the received signals, the receiver means including RF circuit elements ( 11 ) for downconversion of the received signal frequency to a lower frequency known as the first intermediate frequency. According to the invention, the receiver means ( 4 ) include sampling means ( 12 ) performing sampling at the intermediate frequency, at least two digital mixer means ( 13 ) for selective filtration of desired frequency bands from the sampled signal, and processor means ( 14 ) for parallel signal processing at the desired frequency bands.

The invention relates to a radiosonde system according to the preambleof claim 1.

The invention also relates to a radiosonde system receiver and a signalprocessing method used in a radiosonde receiver.

Weather observations in the upper atmosphere are carried out using aradiosonde attached to a sounding balloon. Typically, the soundingballoon is a rubber balloon that is filled with hydrogen (or helium) gasand is dimensioned to elevate a radiosonde as high as up to 40 km. Aradiosonde comprises a radio-frequency transmitter and variousmeasurement equipment for registration of atmospheric phenomena. Thevariables most commonly measured are pressure, humidity and temperature(known as the PTU measurement from words Pressure, Temperature,Humidity) as well as wind speed and direction.

Wind measurement is based on the assumption that the sounding balloonmoves along with the wind in the atmosphere at the same speed as thewind. Hence, the measurement task of wind speed and direction can beperformed by measuring the radiosonde movement. This can beaccomplished, e.g., by means of navigation systems, most common of thembeing Loran-C and GPS.

As a result of the sounding session, a profile is compiled indicatingthe PTU measurement values and wind data at different heights in theatmosphere.

A drawback of the prior art is that a conventional sounding sessiontakes about 2 h, whereby the maximum distance of the radiosonde from thegroundborne sounding station can be, for instance, 200 km. Hence, highdemands are set on the radiosonde battery and overall performance. Thebatteries are purpose-designed special types and the sonde performanceis improved by using a directional antenna as the receive antenna of theground station.

Both the radiosonde battery and the directional receive antenna arerelatively expensive elements. If the system could be adapted to usecommercially available batteries available at a reasonable price and anomnidirectional antenna, substantial cost savings would result.

In addition to equipment cost and performance, also the size ofequipment may in certain cases become a limiting factor. The bestexample of this complication is met in the so-called dropsonde soundingsthat are performed in order to investigate the development ofhurricanes, for instance. Herein, plural radiosondes are dropped from anaircraft at given intervals so that a number of radiosondes aresimultaneously airborne. The tracing of radiosondes is performed using aplurality of single-channel radiosonde units. The optimum solution formore efficient use of space and minimized weight of equipment aboard theaircraft would be a small-size multichannel radiosonde receiver.

In conventional radiosonde receivers, the signal is not sampled at theintermediate frequency as is the case in the present embodiment, butinstead, the digital modulation is decoded using a modem circuit. Thissolution complicates later digital processing of the signal in thereceiver. Multichannel receiver embodiments have not been hereto knownin the art.

It is an object of the present invention to provide an entirely noveltype of radiosonde system, radiosonde system receiver and a signalprocessing method for use in the receiver of a radiosonde system, all ofthese making it possible to overcome the above-described problems of theprior art.

The goal of the invention is achieved by way of using a single-channelor a multi-channel digital receiver as is appropriate for the needs of agiven application.

More specifically, the radiosonde system according to the invention ischaracterized by what is stated in the characterizing part of claim 1.

Furthermore, the radiosonde system receiver according to the inventionis characterized by what is stated in the characterizing part of claim3.

Still further, the signal processing method according to the inventionis characterized by what is stated in the characterizing part of claim5.

The invention offers significant benefits.

The problems associated with the radiosonde system price, size andperformance can be solved using a multichannel digital receiveraccording to the invention with specific features optimized forradiosonde use.

A digital receiver facilitates the use of effective error correctionalgorithms, whereby the transmitter signal successfully detected by thereceiver can have a signal quality and strength substantially lower thanwhat has been possible in the prior art.

As a result, the receive antenna can be made simpler and cheaper.Respectively, the transmit power level of radiosondes can be lowered,whereby the use of cost-effective commercially available batteriesbecomes feasible.

In the following, the invention is examined with the help of exemplaryembodiments by making reference to the attached drawings wherein

FIG. 1 shows schematically an embodiment of the system according to theinvention;

FIG. 2 shows a block diagram of a digital receiver according to theinvention suitable for use in a radiosonde system; and

FIG. 3 shows a flow diagram of the signal processing method according tothe invention;

Referring to FIG. 1, a radiosonde system shown therein comprises aradiosonde 1 that is conventionally elevated to the upper atmosphere bya gas-filled (using helium or hydrogen gas) balloon. Typically, thetransmitter of radiosonde 1 sends to the ground station a digitallymodulated 400 MHz signal that typically conveys pressure, humidity andtemperature data. Accordingly, radiosonde 1 includes all the necessaryequipment for acquisition of weather and position data and a transmitterfor sending this information to further processing either at a groundstation or, e.g., at a receive station located in an aircraft. Inaccordance with the invention, the transmitter signal can be received byan omnidirectional antenna 2 inasmuch as the novel receiver offersimproved sensitivity in combination with the use of digital modulationand error correction methods. Typically, the position data signal of theradiosonde 1 is received by a separate antenna 3. Both signals receivedby antennas 2 and 3 are taken to a digital receiver 4 for furtherprocessing. Further processing of weather data takes place in theradio-frequency module 6 of the receiver and radiosonde position data isprocessed in position data module 5. Data streams from both modules areprocessed further in a computing module 7, whereupon the radiosonde datasignal is passed to the ground station end processing equipment 8. Theposition data can be obtained equally well with the help of the GPSsystem or Loran-C or any other equivalent navigation system.

In FIG. 2 is shown the basic construction of a digital receiver suitedfor use in a radiosonde system. With the help of different front-endmodules 11, the receiver is responsive to data received from an antenna10 at different frequency bands. For instance in Europe, the followingfrequency bands are allocated for radiosonde use:

-   -   400.15-406 MHz    -   1668.4-1700 MHz

In the 400 MHz receiver shown in FIG. 1, the signal is firstdownconverted in the RF front-end module 11 to a first intermediatefrequency (IF). Next, the signal is sampled by an analog-digitalconverter 12 and again downconverted to a second intermediate frequencywith the help of a digital mixer 13 (DDC). The receive signal taken tothe 1680 MHz receiver may be downconverted several times prior tosampling. Otherwise this receiver performs signal processing in the samefashion as a 400 MHz receiver.

A multichannel system can be implemented using a plurality of digitalmixers 13. Then, the input signal of converter 12 may cover, e.g., theentire 400 MHz radiosonde frequency band, whereby selective channelfiltration takes place with the help of digital mixers 13. Furtherprocessing of the data signal takes place by means of a signal processor14. With the help of a front-end processor 15, the signal is processedfurther for transmission over an LAN (Local Area Network) connection 16or a serial port 17 in order to make the weather and position dataaccessible in a normal computer environment such as a Windows, NT or aLinux operating system and/or software running on these. In practice,the front-end processor 15 is a conventional PC running on suitablesoftware such as the NT operating system.

The block diagram of FIG. 3 shows some details of end processing in onechannel.

As shown in FIG. 3, the sampled signal is digitally downconverted 31 ina mixer 13. A signal processor 14 in turn performs the following stepsof the method:

-   -   automatic frequency control 32,    -   demodulation 33,    -   channel equalization 34 compensating for the nonideal behavior        of the channel,    -   error correction 35, and    -   error check 36.

Finally, front-end processor 15 takes care of sending 37 the data to anend-user.

Generally, the center frequency of the radiosonde signal is monitoredand changes in this center frequency are automatically compensated for,thus allowing the use of a low-cost oscillation in the sonde itself.However, center frequency monitoring may be omitted in conjunction withthe present invention provided that the oscillator of the radiosonde isof a sufficiently high quality.

The signal is demodulated by means of a signal processor. Transmissionover the data channel takes place by digital modulation (e.g., GMSK).

After demodulation, the data signal is corrected using thestate-of-the-art techniques. In conjunction with a digital data signal,such modem error correction methods as the Reed-Solomon coding schemeare available.

As error correction algorithms are capable of correcting only a limitednumber of errors, further checking of data integrity is necessary usingverification of checksums, for instance. To this end, the data iscomplemented with one or more checksum algorithms suitable forindicating the integrity of received data. Plural different algorithmsare available for checksum computations.

The channel equalization may also be performed using an a priori knowntraining character sequence. Then, the transfer function of thetransmission channel is computed with the help of the known charactersequence and the receiver character sequence, whereby signal correctionis possible using the thus computed transfer function. While channelequalization is not a mandatory operation as to the function of thepresent invention, it may be advantageously used for improving thesystem performance.

Signal processor 14 transmits the data further to the server process ofthe front-end processor 15 that in turn distributes the data to end-userprocesses over a local area network 16.

1. A radiosonde system comprising at least one radiosonde (1) with meansfor acquisition of position and/or weather data and means for downstreamtransmission of said data over a radio-frequency path, signal capturemeans (2, 3) for reception of said weather and/or position data, andreceiver means (4) for processing said received signals, the receivermeans including RF circuit elements (11) for downconversion of saidreceived signal frequency to an intermediate frequency suitable forsampling, characterized in that said receiver means (4) include samplingmeans (12) for said sampling at said intermediate frequency, at leastone digital mixer means (13) for selective filtration of desiredfrequency bands from the sampled signal, and processor means (14) forparallel signal processing at said desired frequency bands.
 2. Theradiosonde system of claim 1, characterized in that the system includespost-processor means (15, 16, 17) for display of said weather andposition data in a PC environment, for instance.
 3. A radiosonde systemreceiver (4) for reception of position and/or weather data fromradiosondes, the receiver (4) comprising signal capture means (2, 3) forreception of said weather and/or position data, and means (4) forprocessing said received signals, the means including RF circuitelements (11) for downconversion of said received signal frequency to anintermediate frequency suitable for sampling, characterized in that saidreceiver means (4) include sampling means (12) for said sampling at saidintermediate frequency, at least one digital means (13) for selectivefiltration of desired frequency bands from the sampled signal, andprocessor means (14) for parallel signal processing at said desiredfrequency bands.
 4. The radiosonde system receiver (4) of claim 3,characterized in that the receiver (4) includes post-processor means(15, 16, 17) for display of said weather and position data in a PCenvironment, for instance.
 5. A signal processing method for use in aradiosonde system receiver (4), the method comprising the steps ofreceiving by signal capture means (2, 3) a signal transmitted by aradiosonde (1), and downconverting the received signal to anintermediate frequency suitable for sampling, characterized in thatsampling is carried out at said intermediate frequency, selectivefiltration of desired frequency bands is performed for the sampledsignal, and parallel signal processing is performed at said desiredfrequency bands.
 6. The signal processing method of claim 5,characterized in that said sampled signal (30) is downconverted bydigital means (31).
 7. The signal processing method of claim 5,characterized in that said signal is tracked by using automaticfrequency control (32).
 8. The signal processing method of claim 5,characterized in that channel equalization (34) is carried out for saidsignal in order to compensate for a nonideal behavior of the datatransmission channel.
 9. The signal processing method of claim 1,characterized in that said signal is processed using error correction(35) and checksum verification (36).
 10. The signal processing method ofclaim 6, characterized in that said signal is tracked by using automaticfrequency control (32).
 11. The signal processing method of claim 6,characterized in that channel equalization (34) is carried out for saidsignal in order to compensate for a nonideal behavior of the datatransmission channel.
 12. The signal processing method of claim 7,characterized in that channel equalization (34) is carried out for saidsignal in order to compensate for a nonideal behavior of the datatransmission channel.